Light-emitting diodes and laser diodes have heretofore been known as typical surface light-emitting elements. Light-emitting diodes rely on a light-emitting phenomenon where light is generated by the recombination of holes and electrons as carriers are injected into the PN or PIN junction formed on a compound semiconductor (GaAs, GaP, GaAlAs, etc.) by forward-biasing the junction. The laser diode, on the other hand, has a construction in which a waveguide is provided inside the light-emitting diode. When a current of a level exceeding a given threshold value is fed, the injected electron-hole pairs increase, causing a reversed distribution that leads to the amplification (gain) of photons due to stimulated emission. With this, laser oscillation takes place as the light generated by parallel reflex mirrors, such as planes of cleavage, is fed back again to the active layer. As a result, laser light is emitted from the end face of the waveguide.
Negative resistance elements having a light-emitting function (light-emitting thyristors, laser thyristors, etc.) are also known as light-emitting elements having a light-emitting mechanism similar to light-emitting diodes and laser diodes. Light-emitting thyristors are of the PNPN structure formed on a compound semiconductor as described above, and have been commercially available as silicon thyristors. These devices are described in detail in pp. 167-169, "Light-emitting Diodes" (edited by Masaharu Aoki; Kogyo Chosakai Publishing Co., Ltd.), for example. The basic construction of the negative resistance element having a light-emitting function (hereinafter referred to as light-emitting thyristor is exactly the same as that of the thyristor in that a PNPN structure is formed on an N-type GaAs substrate. Its current-voltage characteristic exhibits exactly the same S-shaped negative resistance characteristic as with the thyristor.
The present applicant has already disclosed self-scanning type light-emitting devices using a surface light-emitting type thyristor (hereinafter referred to as surface light-emitting thyristor) in his patent applications, such as Japanese Laid-Open Patent Publication No. Hei-2(1990)-263668, "Light-emitting device"; Japanese Laid-Open Patent Publication No. Hei-2(1990)-212170, "Light-emitting element array and method of driving same"; Japanese Laid-Open Patent Publication No. Hei-3(1991)-55885, "Light-emitting and light-receiving module"; Japanese Laid-Open Patent Publication No. Hei-3(1991)-200364, "Method of reading optical signals and switching element array to be used for same"; Japanese Laid-Open Patent Publication No. Hei-4(1992)-23367, "Light-emitting device"; and Japanese Laid-Open Patent Publication No. Hei-4(1992)-296579, "Method of driving light-emitting element array".
Surface light-emitting elements, such as surface light-emitting diodes and surface light-emitting thyristors, have a problem of poor external light emission efficiency because the light-emitting center is located beneath the electrode for injecting current, making the electrode itself a light shielding layer. This problem will be described in the following, taking the surface light-emitting thyristor as an example.
FIGS. 1A and 1B are a cross-sectional and plan views, respectively, showing a conventional surface light-emitting thyristor of the mesa type PNP structure. Note that these drawings are shown schematically to facilitate the understanding of the construction. This surface light-emitting thyristor comprises an N-type semiconductor layer 24 formed on an N-type semiconductor substrate 1, a P-type semiconductor layer 23, an N-type semiconductor layer 22, a P-type semiconductor layer 21, an anode electrode 40 formed in such a manner as to make ohmic contact with the P-type semiconductor layer 21, and a gate electrode 41 in such a manner as to make ohmic contact with the N-type semiconductor layer 22. Though not shown in the figure, a cathode electrode is provided on the bottom surface of the substrate 1. On the entire structure shown in FIG. 1A provided is an insulating film (not shown) made of a light-transmitting, insulating material, on which an Al wiring 140 (see FIG. 1B) is provided. In the insulating film provided is a contact hole C for electrically connecting the electrode 40 and the Al wiring 140. Another contact hole (not shown) is provided in the insulating film on the gate electrode 41 for connecting the electrode to another Al wiring.
In this surface light-emitting thyristor of the PNPN structure, most of the current fed from the anode electrode 40 flows directly downward, as shown by an arrow in FIG. 1A (indicated by I.sub.1.) The light-emitting center of the gate layers 22 and 23 therefore lies beneath the electrode 40. Because of this, light is shielded by the electrode 40 itself and by the Al wiring 140, lowering the external light emission efficiency.
The amount of light emitted is large in areas near the electrode 40 because of the high density of injected current there, while the corresponding amount is reduced in areas far away from the electrode 40 because the density of injected light becomes smaller. This is one of factors contributing to lowered external light emission efficiency.
Another factor responsible for lowered external light emission efficiency is that part of current injected from the anode electrode 40 flows going round to the gate electrode 41 (indicated by I.sub.2). The light emitted by the current I.sub.2 cannot be used because it is inclined toward the gate electrode 41. As a result, the amount of light obtained in areas near the anode electrode 40 is reduced. Japanese Examined Patent Publication No. Hei-5(1993)-25189 discloses a conventional technique of enhancing external light emission efficiency in which the shape of the light-emitting surface of each light-emitting diode in a monolithic light-emitting diode array is made into a U shape by drawing current-feeding wiring to the central part of the light-emitting surface of each light-emitting diode. In this prior-art, however, external light emission efficiency cannot be improved materially because the light-emitting center still lies beneath the electrode.
Japanese Laid-Open Patent Publication No. Hei-4(1992)-259263 discloses a technique of improving light emission efficiency in which the light-emitting region of the active layer in a semiconductor light-emitting element is expanded to a sufficient degree so that the light from the light-emitting region can be extracted without shielding with the electrode on the light extracting side. But the structure and manufacturing method of semiconductor in this technique are considerably complex.
Japanese Laid-Open Patent Publication No. Hei-5(1993)-211345 also discloses a technique of improving external light emission efficiency in a surface light-emitting diode that emits light from a light extracting surface by stacking P-type semiconductors and N-type semicondutors on a substrate, forming a light extracting surface on the topmost part of the stacked semiconductors, and feeding a working current between an upper electrode installed on the light extracting surface and a lower electrode installed on the bottom surface of the substrate, thereby the concentration of impurities at portions other than the portion beneath the upper electrode in a plane parallel with the light extracting surface to form a current control layer that allows the working current to flow easily in the portions other than the portion beneath the upper lectrode. In this technique, however, manufacturing process becomes complex.